As a method for realizing stable feedback control without increasing the ripple component of the output voltage itself in the ripple control system, for example, there is the scheme using a CR integrator described in Takashi Kabeshima and 3 others: “Control characteristics of voltage buck-type converter by means of hysteresis PWM control using CR integrator,” Denshi Joho Tsushin Gakkai Ronbunshi [IEICE Papers], published by The Institute of Electronics, Information and Communication Engineers, May 2006, Vol. J89-B, No. 5, pp. 664-672. In this scheme, a signal similar to the ripple current flowing through inductor Lo is retrieved by means of a CR integrator connected in parallel with inductor Lo, and the retrieved signal is superimposed on the output feedback voltage.
FIG. 19 is a diagram illustrating a constitutional example of the switching power supply device of the ripple control system using the ripple signal retrieved by the CR integrator.
In the switching power supply device shown in FIG. 19, the CR integrator made up of a series circuit of resistor R35 and capacitor C35 is connected in parallel with inductor Lo. At capacitor C35, a ripple voltage similar to the ripple current flowing through inductor Lo is generated. The connection node between resistor R35 and capacitor C35 is connected to the connection node between resistors R31 and R32 via capacitor C34, so that the ripple voltage generated at capacitor C35 is superimposed on output feedback voltage VFB.
FIG. 20 is a diagram illustrating an example of the signal waveforms at the various portions in the switching power supply device shown in FIG. 19.
Even when the ripple voltage of output voltage Vout is relatively low (FIG. 20(B)), ripple voltage Vrp with a sufficient amplitude at output feedback voltage VFB is obtained (FIG. 20(C)). When the trough of output feedback voltage VFB is less than reference voltage Vref, gate-source voltage Vgs of MOS transistor MH goes to the high level for a prescribed time (FIG. 20(A)). The switching power supply device shown in FIG. 19 superimposed ripple voltage Vrp with an appropriate amplitude on output feedback voltage VFB, so that even when the ripple voltage of output voltage Vout is relatively low, it is still possible to have stable operation of the control system, which is advantageous.
However, in the ripple control system, because control is performed so that the peak and trough of the ripple component are in agreement with reference voltage Vref, so that the amplitude of ripple voltage Vrp is not very large. If the amplitude of ripple voltage Vrp is too large, the deviation between the DC level defined by reference voltage Vref and the DC level of actual output voltage Vout increases, which is undesirable for guaranteeing the DC precision of output voltage Vout. Consequently, the amplitude of ripple voltage Vrp must be set within an appropriate range in consideration of the accuracy required for output voltage Vout. However, in this case, in consideration of the overall switching frequency requirement, the time constant of CR integrator (R35, C35) may have to be made larger. If the capacitance of the capacitor were, e.g., several thousand pF, it would be difficult to form it on a semiconductor chip, and it would have to be assembled as a discrete element on the substrate.
Usually, in the switching power supply device shown in FIG. 19, resistor R35 and capacitors C34, C35 must be assembled as discrete elements on a substrate, so that the size of the substrate increases, as does the cost of assembly of the elements, which is undesirable.